Electrical earth borehole logging is well known and various devices and various techniques have been described for this purpose. Broadly speaking, there are two categories of devices used in electrical logging devices. In the first category, a transmitter (such as a current electrode) is uses in conjunction with a diffuse return electrode (such as the tool body). A measured electric current flows in a circuit that connects a current source to the transmitter, through the earth formation to the return electrode and back to the current source in the tool. In inductive measuring tools, an antenna within the measuring instrument induces a current flow within the earth formation. The magnitude of the induced current is detected using either the same antenna or a separate receiver antenna. The present disclosure belongs to the first category.
With tools in tools of the first category, it is desirable to have an imaging technique that is minimally affected by formation current leakage to the tool mandrel and tool standoff from the formation, especially, under conditions when formation becomes conductive (below 10 ohmm). Typically, one or more transmitters are disposed on the tool mandrel and impart electrical current into the earth formation. A return current is received from the earth formation by one or more electrodes on a pad. However, at higher frequencies of operation, the standoff distance between the tool mandrel and the earth formation may be sufficiently large as to degrade the quality of the signal received through the return electrode(s) on the pad. Additionally, borehole curvature and diameter may limit the use of some pad configurations due to prohibitively large pad-to-formation standoff distances and/or and inability to convey the pad in the borehole. This disclosure addresses tool configurations for reducing current leakage from the formation to the tool mandrel and reducing the effects of standoff on resistivity imaging.